net/virtio: return error when device_opts arg is NULL

This fixes CID 1407222.

Fixes: 9711cd0d ("net/virtio: add failover support")
Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>